Whereas most animals shed and regrow hair seasonally, humans shed about 100 strands of hair out of about 100,000 strands and about 100 strands regrow every day, thus always maintaining a similar number.
The hair growth cycle consists of anagen, catagen and telogen phases. During the anagen phase, hair growth is promoted as cell division occurs actively in the hair papilla. Hair grows only in this stage. Considering that the anagen is about 3-5 years for men and about 4-6 years for women, about 80-85% of hair is in the anagen phase. In the catagen phase, which lasts about 3-4 weeks, the cell division declines gradually. Lastly, in the telogen phase, the hair papilla is withdrawn and the hair separated from the capillary vessel and simply stuck in the scalp. This lasts about 3 months and the hair in the telogen phase is easily lost upon physical stimulation.